An office machine such as a printer or a scanner is widely used in the office. For achieving diversified functions and integrating more functions, the office machine is usually equipped with a post-processing apparatus. By the post-processing apparatus, plural documents outputted from the printer or the scanner can be automatically stapled or punched in order to enhance the working efficiency. Regardless of whether any task is performed, the document outputted from the office machine with the post-processing apparatus is firstly introduced into a first sheet tray of the post-processing apparatus, and finally ejected to a second sheet tray outside the office machine through the a sheet ejection device.
Hereinafter, the structure of a conventional post-processing apparatus will be illustrated with reference to FIGS. 1 and 2. FIG. 1 is a schematic perspective view illustrating a conventional post-processing apparatus. FIG. 2 is a schematic side view illustrating the conventional post-processing apparatus.
Please refer to FIGS. 1 and 2. The post-processing apparatus 1 comprises a first sheet tray 11, a second sheet tray 12, and a sheet ejection device 13. The first sheet tray 11 has a first end 11a and a second end 11b. The sheet ejection device 13 comprises a ring-shaped conveyor belt 13a, a sheet ejection arm 13b, a first roller 13c, and a second roller 13d. 
The second sheet tray 12 is located beside the second end 11b of the first sheet tray 11. The first roller 13c is disposed under the first end 11a of the first sheet tray 11. The second roller 13d is disposed under the second end 11b of the first sheet tray 11. The first roller 13c and the second roller 13d are enclosed by the ring-shaped conveyor belt 13a. In addition, the sheet ejection arm 13b is protruded from an outer surface of the ring-shaped conveyor belt 13a. 
Hereinafter, the operations of the conventional sheet ejection device will be illustrated with reference to FIGS. 3A, 3B and 3C. FIG. 3A is a schematic side view illustrating the conventional post-processing apparatus, in which the sheet ejection device is in an initial position. FIG. 3B is a schematic side view illustrating the conventional post-processing apparatus, in which plural paper sheets are pushed by the sheet ejection device. FIG. 3C is a schematic side view illustrating the conventional post-processing apparatus, in which the tail edges of the plural paper sheets are moved to the second end of the first sheet tray.
After plural paper sheets S are introduced into the first sheet tray 11 through the first end 11a of the first sheet tray 11, the paper sheets S are sequentially stacked on the first sheet tray 11.
Before a sheet-ejecting operation is performed, the ring-shaped conveyor belt 13a is in a static state. Under this circumstance, the sheet ejection arm 13b is stayed in an initial position, which is located under the first sheet tray 11 (see FIG. 3A).
Once the sheet-ejecting operation is started, the second roller 13d is rotated. Upon rotation of the second roller 13d, the ring-shaped conveyor belt 13a and the sheet ejection arm 13b are simultaneously and circularly moved, and the first roller 13c is driven to be rotated.
Consequently, the sheet ejection arm 13b moved upwardly from the initial position. In addition, the sheet ejection arm 13b is exposed to the surface of the first sheet tray 11 from the first end 11a of the first sheet tray 11, so that the tail edges of the plural paper sheets S are pushed by the sheet ejection arm 13b (see FIG. 3B).
The sheet ejection arm 13b is continuously moved toward the second end 11b of the first sheet tray 11, so that the plural paper sheets S are moved in the direction toward the second sheet tray 12.
When the sheet ejection arm 13b and the tail edges of the plural paper sheets S reach the second end 11b of the first sheet tray 11 (see FIG. 3C), a pushing force in the direction x is exerted on the plural paper sheets S. Moreover, due to the gravity of the plural paper sheets S, the plural paper sheets S are fallen down to the second sheet tray 12. The sheet ejection arm 13b is continuously moved downwardly, and finally returned to the initial position.
The sheet ejection device of the post-processing apparatus, however, still has some drawbacks. For example, when the tail edges of the plural paper sheets reach the second end of the first sheet tray, the plural paper sheets have been completely detached from the first sheet tray, but the pushing force in the direction x is still exerted on the plural paper sheets. If no instantaneous downward force is exerted on the plural paper sheets to allow the tail edges of the plural paper sheets to be quickly introduced into the second sheet tray, the plural paper sheets may easily fly in the sheet-pushing direction of the sheet ejection arm. Under this circumstance, the plural paper sheets fail to be uniformly aligned with each other.
If the moving speed of the sheet ejection arm is increased to enhance the overall working efficiency, the above problems become more serious.
Therefore, there is a need of providing a post-processing apparatus with an improved sheet ejection device in order to eliminate the drawbacks encountered from the prior art.